GEOLOGY | FOSSILS | key fossil groups
Foraminifera (forams) are simple, single-celled, protozoans with a protective, pitted shell known as a test. Another characteristic is the network of granular pseudopodia that stream from the cell wall, e angling the individual to move and capture food. They vary in size anywhere from 1 μm to 110 mm. They are usually between about a half and one millimetre long. The nomenclature is derived from the characteristic foramen, an opening or tube that interconnects all the chambers of the test, translating as 'hole bearers'. Foraminifera are of the phylum: Retaria and the class: Granuloreticulosea.
Fossilised tests are found in wide range of sedimentary rocks as old as the earliest part of the Cambrian Period and throughout the Phanerozoic (to present). Forams are of particular biostratigraphical and palaeoenvironmental value.
There are different schemes of classification of forams. However, to identify species and higher categories of foraminifera, shell morphology and imerallogy are the keystone. The tests of foraminifera fall into one of three categories:
Agglutinated tests are made from cemented particles found by the Foraminifera in the immediate environment eg. grains of mica, quartz; or possibly bioclastic fragments like spicules; or fragments coccoliths or even foram tests.
The specimen in Image II (A) is of the agglutinated class of test.
Organic tests are made from the chitin-like polysaccharide, tectin. It forms a gelatinous membrane which envelopes the foram.
Allogromiid foraminifera use tectin to produce their relatively simple, single-chambered tests. They are found as both marine and freshwater forms, and are the oldest forms known from the fossil record.
Secreted biogenic calcareous tests fall into three categories:
1. Porcellaneous. (Image. II B).
Formed of small, randomly orientated crystals of high-magnesium calcite producing a smooth, white shell.
2. Hyaline. (Image. II D).
Hyaline tests are glassy as they comprise large, low-magnesium calcite crystals.
3. Microgranular. (Image. II C).
Test described as microgranular are formed from homogeneously sized, tightly packed crystalline calcite.
In some types of foraminifera, the chambers have a simple spiral assemblage and take a number of forms. The simplest is a sphere or a tube with an aperture at one end (Image. IV A): these are known as tubular. Chambers may be added in a single row, like a string of beads, termed: uniserial (Image. IV B). Those with two rows of chambers are called biserial (Image. IV C) and those with three rows, triserial (Image. IV D).
Foram chambers may be added in complex spiral forms. Planispiral (Image IV. E) tests resemble a Chelsea Bun whilst trochospiral tests (Image IV J) are like a tiny snail. In streptospiral tests (Image. IV I), each chamber is half a whorl.
In some types of foraminifera the chambers are complex. eg planispiral and trochospiral forms.
Image. IV F: Milioline, G: Planispiral evolute. & H: Planispiral involute.
APERTURE & Ornament
Apart from chamber construction, the shape and position of the aperture is a significant feature that characterises foram groups. In Image V we see the principal types of aperture:
A. open end of tube
B. terminal radiate
C. terminal slit
E. loop shaped
G. interiomarginal multiple
H. areal crbrate
I. with phialine lip
J. with bifid tooth
K. with umbilical teeth
Test surface ornament may include ribs, spines or its surface may be described as punctate, rugose, striate or reticulate. See Image. VI
Suborder Spirillinina has a test constructed of an optically single
crystal of calcite
The Suborder Involutina have a two chambered test composed of aragonite
the Suborder Silicoloculinina as the name suggests has a test composed of silica
The Suborder Carterinina secretes spicules of calcite which are subsequently cemented together to form the test
The pyramids in Gizeh, Egypt, are in part built of a Palaeogene limestone which contains huge numbers of Nummulites gizehensis, a large foraminifera that grew to several centimetres across.
McKleish, R. 1986. Geological Science (Blackie, Glasgow).
Wilkinson, I P. 1997. Foraminifera: fossil focus. (Nottingham, UK: British Geological Survey.)